Volumina Jurassica, 2010, VIII: 103–130

A precise ammonite biostratigraphy through the Kimmeridgian- -Volgian boundary beds in the Gorodischi section (Middle Volga area, Russia), and the base of the Volgian Stage in its type area

Mikhail Rogov 1

Key words: Kimmeridgian/Volgian boundary, Volgian SSSP, ammonite faunal horizons, Boreal-Tethyan correlation.

Abstract. A detailed study of the ammonite faunal horizons of the uppermost Kimmeridgian–Lower Volgian of the Middle Volga area (Russia) was undertaken at the most complete and well-known Gorodischi section. This section shows a complete transition from the Kimmeridgian to the Volgian and is a possible SSSP candidate for the Volgian Stage. Sixteen faunal horizons have been established between the top of the Eudoxus Zone and the top of the Lower Volgian. Revised descriptions of existing horizons (especially in the Kimmeridgian) are given and description of new horizons: cf. anglicum, aff. rebholzi, zeissi, ilowaiskii, cf. praecursor, sokolovi, pavida, “Franconites”. Oscillations in the ratios of ammonites with different affinities from horizon to horizon reflect short-term changes in the climate and/or palaeogeography. The presence of ammonites with Submediterranean affinities throughout the studied interval enables correlations to be made with the Tethyan ammonite succession. The evolution of the eudemic Subboreal lineage of Sarmatisphinctes has been analysed, and a succession of species through the complete Autissiodorensis Zone has been recognised. This comprises, in ascending order, S. cf./aff. subborealis, S. subborealis, S. zeissi, S. fallax and S. ilowaiskii. Two of the species described, Sarmatisphinctes zeissi and S. ilowaiskii, are new.

Geological setting shown that it contains no biostratigraphically significant gaps (Kiselev, Rogov, 2005). The ammonite infrazonal biostrati­ The Gorodischi section is one of the most famous and graphy of several additional sections in the Kimmeridgian- best-studied sections in the Russian . It is located -Volgian (Ki/Vo) transition beds were also studied. These are high on the right bank of the River Volga, c. 25 km north of mostly located in the Middle Volga area and, farther south, Ul’anovsk (Figs 1, 2). It was chosen as the lectostratotype on the borders of the Peri-Caspian Lowland (Fig. 1). for the Volgian Stage by Gerasimov and Mikhailov (1966). A recent study of the Ki/Vo boundary beds at Gorodi- Following a suggestion by Cope (1996) that secondary schi (Rogov et al., 2006) included the collection of magne- standards should be accepted, Zakharov (2003) proposed the tostratigraphic, geochemical and microfossil data that is cur- Gorodischi section as the Secondary Stratotype Section and rently being analysed. A detailed description of the section Point (SSSP) for the Volgian Stage. Although the Volgian has been published (Rogov, Kiselev, 2007): a log of the sec- succession there is thin and condensed, recent studies have tion together with the ammonite data is given here (Fig. 3).

1 Geological Institute, Russian Academy of Sciences, Pyzhevskii Lane 7, Moscow 119017, Russia; e-mail: [email protected] 104 Mikhail Rogov

60 terrestrial conditions

shallow shelf seas

ocean basin

Kimmeridgian- 50 -Volgian sections of the European Russia

Middle-Russian Sea

4 3 2 Moscow Ul’anovsk 40 1 Fig. 1. Palaeogeography of the Middle-Russian Sea 6 during the Kimmeridgian-Volgian transition (modified after Thierry, 2000) Kimmeridgian-Volgian sections of the European Rus- 30 sia mentioned in article: 1 – Berd’yanka (51o25’55” N; 55o25’07” E), 2 – Gorodischi (54o34’55” N; 48o24’56” E), 3 – Polevye-Bikishiki (55o05’40” N; 47o29’21” E), 4 – Murzicy (55o18’15” N; 46o11’40” E), 5 – Isady (56o04’40” N; Neotethys 45o07’13” E), 6 – Elton (49o8’56” N; 46o50’49” E) 0 200 km

Fig. 2. Uppermost Kimmeridgian of the Gorodischi section (photo by A. Manikin, 2005)

Fig. 3. Ammonite succession through the Kimmeridgian- -Volgian transition of the Gorodischi section A – stratigraphical log (bed numbers after Rogov, Kiselev, 2007); B – ammonite ranges; C – ammonite assemblages. Boreal ammonites (1–2): 1 – Cardioceratidae (Nannocardioceras, Hoplocardioceras, Euprionoceras); 2 – Dorsoplanitidae (Eosphinctoceras, Subdichotomoceras); 3 – Boreal Ochetoceratinae (Suboxydiscites); Subboreal ammonites (4–5): 4 – Virgatitidae (Sarmatisphinctes, Ilowaiskya, “Pseudovirgatites”); 5 – Aulacostephanidae (); Submediterranean ammonites (6–8): 6 – Aspidoceratidae (Aspidoceras, Schaireria, Sutneria); 7 – Oppeliidae (Neochetoceras, Taramelliceras, Ochetoceras, Paralingulaticeras (Rogoviceras)); 8 – Lithacoceratinae (Discosphinctoides, ?Franconites). Ammonites with Subtethyan affinities marked by bold; Boreal ammonoids are underlined; N – number of specimens A precise ammonite biostratigraphy through the Kimmeridgian-Volgian boundary beds... 105

A B C

FAUNAL HORIZON THICKNESS [m] SUBZONE ZONE LITHOLOGY BED NO. SUBSTAGE

1/6-7 Pseudovigratites N = 79 puschi spp 1/8a,b spp Schaireria sp. 1/9a-c neoburgense sp. N = 224 os tatum Tenuic Ps eudos c y thic a I. pseudoscythica 1/10 ? Franconites N = 43 “Franconites” 1/11 Ilowaiskya pavida Sphinctoceras

1/12 Fontannesiella Ilowaiskya sokolovi ? I. pavida Paralingulaticeras (Rogoviceras) efimovi N = 55 spp. ? Danubisphinctes sp. Eosphinctoceras I. sokolovi Sutneria asema N = 47 1/13a,b Sok olov i N = 251 P. (R.) efimovi “Pseudovirgatites” puschi

LOWER VOLGIAN 1/14

N. steraspis Schaireria neoburgense N = 205 1/15 Klimov i “Pseudovirgatites” tenuicostatum N. cf. praecursor 1/16-18 Aulacostephanus Ilowaiskya pseudoscythica Sarmatisphinctes 9/43= 1/19 zio N = 159 ilowaiskii cf. praecursor 9/39-42 cf. franciscanum Ilowaiskya klimovi cf.

Fallax Sarmatisphinctes 9/38 Neochetoceras steraspis

Ochetoceras N = 108 fallax ilowaiskii

9/36-37

9/34-35 Neochetoceras taimyrensis

9/32-33 Taramelliceras cf. Sarmatisphinctes 9/29-31 zeissi Sarmatisphinctes 9/28 N = 100 in Callomon, Cope, 1971, pl. 10, fig. 7) Neochetoceras rebouletianum 9/25-27 Suboxydiscites Sarmatisphinctes subborealis Sarmatisphinctes fallax sp.

9/23-24 Autissiodorensis

subborealis N = 112 circumspinosum

9/22 cf. Sarmatisphinctes Subborealis subborealis 9/19-21 subsidens Nannocardioceras N = 39 cf. Sarmatisphinctes zeissi

UPPER KIMMERIDGIAN 9/14-18 (= Physodoceras anglicum Sarmatisphinctes rebholzi

cf. N = 36 9/13 sp. aff. 9/12 N = 61 ex Neochetoceras

Nannocardioceras volgae 9/10-11 Nannocardioceras volgae Schaireria Sutneria Sutneria aff. rebholzi Nannocardioceras krausei N = 74 2 9/9 N = 48 9/4-6 Nannocardioceras subnudatum N. cf. anglicum 1 gr. Neochetoceras N = 43 9/3 Eudoxus

marly clay beige clay phosphorite nodules

grey clay dark-grey, bituminous clay ammonite accumulations

Ammonites:

1 2 3 4 5 6 7 8 106 Mikhail Rogov

Infrazonal ammonite biostratigraphy Subborealis Subzone Zeiss, 2003 emend. Rogov, herein

The ammonite successions in all the sections studied This subzone is marked by the occurrences in large num­ show mixtures of Suboreal, Boreal and Submediterranean bers of the Subboreal ammonite genus Sarmatisphinctes, taxa which enable detailed long-range correlations within possibly derived from Discosphinctoides, throughout the south- the Kimmeridgian, as well as between the Lower Volgian, east part of the Russian Platform and as far as central Poland. the Arctic Volgian and the Stage. The ammonite The base of the subzone in the area studied is characterized biostratigraphy of the Lower Volgian has been described in by sudden changes in the ammonite assemblages. The rarity detail (Rogov, 2004a); some additional remarks are included of A. autissiodorensis records in the higher parts of subzone here. In contrast, the ammonite biostratigraphy of the Upper and the absence of aulacostephanids in some areas (e.g. Kimmeridgian has been little studied in the area under con- the Elton section) makes the Subborealis Subzone easier to sideration. The ammonite succession has been summarized recognise in the area investigated. Scherzinger and Mitta in a field guide (Rogov, Kiselev, 2007), but without descrip- (2006) have introduced the Volgae Subzone for the same tions of the biostratigraphical units or fossils. The same bed stratigraphical interval on the grounds that “at the present time numbers are used here as in the field guide. it is unclear if the species ‘Divisosphinctes sublacertosus’ and ‘Discosphinctoides subborealis’ are conspecific” (loc. Upper Kimmeridgian cit., p. 233). However, “D. sublacertosus”, as noted by Ilovaisky (in: Ilovaisky, Florensky, 1941), occurs below Eudoxus Zone Neumayr, 1873 Aulacostephanus. Moreover, S. subborealis offers additional advantages as an index-species for the following reasons: The uppermost part of the Eudoxus Zone in the Gorodis- – it has a wide stratigraphical range within the subzone; chi section is characterized by an ammonite succession that – it belongs to the lineage that is used as the basis for is close to those of other Subboreal regions such as England subzonal and zonal subdivision of the Upper Kim- and Extra-Carpathian Poland. The highest ammonite fau- meridgian and a major part of the Volgian; nal horizon of this zone is marked by an absence of aula- – as index species it has priority over the Volgae Subzone costephanids and a strong Boreal influence. Ammonites at proposal. this level are represented chiefly by the small-sized Nanno- Both subzones – the Subborealis and Volgae subzones cardioceras cf. anglicum (Salfeld) (Pl. 1: 1) and are assigned to – were erected (cf. Zeiss, 2003; Scherzinger, Mitta, 2006) the cf. anglicum faunal horizon (Rogov, Kiselev, 2007) which without their boundaries being defined. The Subborealis corresponds to the Nannocardioceras Beds of Callomon and Subzone is here defined as follows: the lower boundary is Cope (1971). On the Dorset coast the Nannocardioceras Beds marked by the FADs of Aulacostephanus autissiodorensis, lie a little below the Flats Stone Band which marks the base of Sarmatisphinctes aff. subborealis, Aspidoceras ex gr. cata- the Autissiodorensis Zone (Cox, Gallois, 1981). launicum and Sutneria aff. rebholzi. The upper boundary is marked by the FAD of Sarmatisphinctes fallax. Autissiodorensis Zone Ziegler, 1962 aff. rebholzi horizon Rogov, nov. Scherzinger and Mitta (2006) assumed that the Fallax Zone should replace the Autissiodorensis Zone in the Stratotype. — Gorodischi section, bed 9/9, light beige to Russian succession on the grounds of priority, and because gray calcareous clays. of the differences in interpretation of the Autissiodorensis Index species. — Sutneria aff. rebholzi Berckhemer [m] Zone by the different authors. The Fallax Zone was proposed (Pls 1: 7–9; 5: 7); the neotype of S. rebholzi designated and more than 20 years before the Autissiodorensis Zone, but the figured by Berckhemer and Hölder (1959, pl. 12: 61) was replacement of the one zone by the other is not convincing stated to have come from the ?Subeumela Subzone (af- because the two zones have different stratigraphical ranges. If ter Berckhemer, Hölder, 1959), but the preservation of the the Autissiodorensis Zone is defined as the interval between specimen suggests that it came from the Ulmensis Subzone, the FAD of Aulacostephanus autissiodorensis and the LAD zio-wepferi β horizon (Schweigert, 1998). Specimens from of Aulacostephanus, this zone can be recognizable over a Russia differ from the neotype of S. rebholzi, and are almost large area. The Fallax Subzone corresponds to the upper identical to those from the basal Autissiodorensis Zone of Eng- part of the Autissiodorensis Zone in East Europe (Russian land referred to as S. rebholzi (Callomon, Cope, 1971, pl. 9: Platform and Extra-Carpathian Poland). 2–4) and from northern Poland (Krause, 1908, pl. 3: 8, 9). A precise ammonite biostratigraphy through the Kimmeridgian-Volgian boundary beds... 107

Characteristic ammonites. — Nannocardioceras krausei proposed by Schweigert et al. (1996), but the known range (Salfeld) [m] (Pl. 1: 2, 3); N. cf. ewae (Malinowska) [m] (in of the species of Sutneria in Germany does not support such the uppermost part only, uncommon); Aulacostephanus autis- a correlation. In Germany Neochetoceras subsidens is ac- siodorensis (Cotteau) [M]; A. undorae (Pavlov) [m]; A. cf./ companied by numerous S. subeumela (op. cit.), and S. reb- aff. eudoxus (d’Orbigny) [m]; A. kirghisensis (d’Orbigny) holzi is only known from the zio-wepferi α-β horizons (i.e. [m] (Pl. 5: 5); A. volgensis (Pavlov) [?m]; Aspidoceras aff./cf. 5 horizons higher), The Setatum Subzone – especially the catalaunicum (Loriol) [M] (Pl. 2: 8); ?Anaspidoceras sp. [M]; ornatum horizon (Tab. 1), is characterized in Germany by S. Neochetoceras cf. subsidens (Fontannes)1 [M] (Pl. 1: 10); Lin- casimiriana (Font.). The stratigraphical range of forms re- gulaticeras sp. [m]; rare Sarmatisphinctes cf./aff. subborealis ferred to herein as S. aff. rebholzi in the Subboreal Realm may (Kutek and Zeiss)2, which is usually characterized by a lower thus be lower than the Swabian records of typical S. rebholzi. rib ratio compared to typical S. subborealis. The presence of rare S. subeumela (Schneid) in bed 9/10 at Comments. — In spite of doubts about the identification Gorodischi attributed to the volgae horizon (see below), and of some of the English and Russian specimens referred to as the presence of S. subeumela above S. aff. rebholzi (Krause, S. rebholzi Berckhemer (see below), this is possibly the best 1908, pl. 3: 6) additionally suggest that S. aff. rebholzi repre- index-form for the horizon. Macroconchiate aspidoceratids are sents a new species independent of true S. rebholzi. usually poorly preserved in that horizon, and Nannocardioceras Geographical range. — Middle Volga area (Gorodischi, krausei (Salfeld), which occurs at this level on the Russian Murzicy sections), England (from Dorset to Wash area), Platform, may have a longer range in England, extending down northern and central Poland. into the uppermost Eudoxus Zone. In the Orenburg area this level is characterized by Aspidoceras catalaunicum, but the lacks aspidoceratid microconchs. Uncommon records volgae horizon Scherzinger and Mitta (2006) of A. catalaunicum in the Pechora area and Subpolar Urals (as subzone) emend. Rogov, herein may be dated to the same horizon. However, these cannot be considered as indicative of a catalaunicum horizon Stratotype. — Gorodischi section. This horizon was not because this lies at a different stratigraphical level in the defined in the stratigraphical log of Scherzinger and Mitta French succession (Hantzpergue, 1989). Anaspidoceras (2006): the highly bioturbated dark-gray clays of bed 9/10 sp. from this horizon in Russia is characterized by the are proposed here as the stratotype. presence of periumbilical tubercles only. Crushed shells Index species. — Nannocardioceras volgae (Pavlov) [m] of such ammonites resemble A. fluegeli (Zeiss, 1994, figs 1, (Pl. 1: 4–6); type (SD by Malinowska, 2001), CNIGR Muse- 2; pl. 1: 1, 2) and other early species of Anaspidoceras, but um, Saint-Petersburg no. 37/312 (re-figured by Malinowska, poor preservation prevents their precise identification. 2001 and herein – Pl. 1: 4); Kimmeridgian (“Acanthicum Correlation. — The occurrence of the index form (mi- Zone” of Pavlov, 1886) of the Gorodischi section, perhaps croconchs) and accompanying macroconchs in the English bed 9/10 (based on lithology and mode of preservation). succession provides a good correlation with the basal beds Characteristic ammonites. — Nannocardioceras volgae of the Autissiodorensis Zone; the presence of comparable (Salfeld) [m] (very common); Aulacostephanus undorae rare Sutneria in the basal part of the Autissiodorensis Zone (Pavlov) [m] (Scherzinger, Mitta, 2006, fig. 6: 2);N. krausei in the J. Kutek collection (Stobnica 36 see Kutek, 1961; cf. (Salfeld) [m] (very rare); Aulacostephanus sp. (= “volongen- Kutek, Zeiss, 1997) and those figured by Krause (1908), sis”3 Khudyaev sensu Scherzinger and Mitta – Scherzinger, provide a correlation with the Polish succession. Records of Mitta, 2006, fig. 6: 3); Aulacostephanus autissiodorensis Neochetoceras cf. subsidens in the Russian sections suggests (Cotteau) [M]; Sarmatisphinctes aff./cf. subborealis (Kutek a correlation with the Late Kimmeridgian in Germany as et Zeiss) [m], Sutneria subeumela Schneid [m] (cf. Geyer,

1 The precise identification of theNeochetoceras is difficult due to their mode of preservation, usually as crushed clayey moulds.Neochetoceras from the uppermost Eudoxus Zone of Gorodischi, sampled during the recent fieldwork, can be assigned toN. acallopistum by comparison with specimens from the basal Autissiodorensis Zone, which are assigned to N. cf. subsidens.

2 This species was referred to the genus Sarmatisphinctes by Scherzinger and Mitta (2006). This interpretation is supported here: the species cf./aff. subborealis–subborealis–zeissi–fallax–ilowaiskii represent a continuous lineage of true Subboreal derivates of Discosphinctoides.

3 The type specimens of A. volongensis, stored at CNIGR Museum (Saint-Petersburg), were considered by M.S. Mesezhnikov (1984) as Upper ammonites. Reevaluation of these specimens (refigured here at Pl. 2: 1a–c), has shown that they could be the microconchiate Aulacostephanus resembling A. undorae (Pavlov). The ammonite figured by Scherzinger and Mitta (2006) under the name A. volongensis could be either microconch or small-sized macro- ­conch resembling A. undorae (Pavlov) or A. autissiodorensis (Cottreau), respectively. Typical A. autissiodorensis (Cottreau) were also found in this horizon. 108 Mikhail Rogov

Table 1 Correlation of the Kimmeridgian-Volgian boundary beds of European Russia and Southern Germany

Kimmeridgian-Tithonian boundary beds of SW Germany (Scherzinger, Schweigert 2003; Schweigert et al., 1996; Schweigert, 2000) Kimmeridgian-Volgian boundary beds of Russian Platform

sub- , sub- sub- stage zone subzone faunal horizon faunal horizon zone zone stage

“Franconites” vimineus VIMI- NEUS

levicostatum pavida SOKOLOVI

MUCRO- NATUM franconicum

sokolovi Moerns- laisackerensis heimensis LOWER TITHONIAN cf. eystettense efimovi

Ruepellianum riedlingense LOWER VOLGIAN (partially) rueppellianum steraspis KLIMOVI HYBONOTUM Riedense eigeltingense cf. praecursor

rebouletianum ilowaiskii

hoelderi Ulmense zio-wepferi α fallax zio-wepferi β siliceus uracensis zeissi BECKERI Setatum ornatum AUTISSIODORENSIS

UPPER KIMMERIDGIAN supinum subborealis UPPER KIMMERIDGIAN minutum Subborealis Fallax volgae Subeumela subsidens aff. rebholzi

Black arrows indicates levels of occurrence of ammonites of the Submediterranean origin in the Ul’anovsk Volga area suggesting po- ssibilities of direct Subboreal/Submediterranean correlation

1969, fig. 4; Scherzinger, Mitta, 2006, fig. 6: 1; the matrix shows In both countries mentioned above, and in Russia, N. vol- that it came from the bed 9/10 – Mitta, 2007, person. commun). gae has a very restricted stratigraphical range and has the po- Comments. — In England, a faunal marker bed identified tential to be the index fossil of a faunal horizon that can in the lowermost Autissiodorensis Zone by Cox and Gallois be used for correlation over much of the Boreal Province (1981) was characterised by a distinctive form of N. aff. an- despite the fact that its range in Poland is still imprecisely glicum (Salfeld) with fine rectiradiate ribbing identified by known. The FAD of N. volgae has been proposed as a mark- Van de Vyver (1986) as N. volgae, and named the Volgae er for the base of the Fallax Zone and the Volgae Subzone Band by Gallois (2000). A similar bed has been recorded in (Scherzinger, Mitta, 2006) but such an interpretation dif- Poland (Kutek, Zeiss, 1997). fers from that accepted herein. Among the pecularities of A precise ammonite biostratigraphy through the Kimmeridgian-Volgian boundary beds... 109 this level in Russia should be noted: a rarity of macroconch Index species. — Sarmatisphinctes subborealis (Kutek et records (similarly in Poland; Kutek, Zeiss, 1997), its devel- Zeiss, 1997) (Pls 1: 11, 12; 5: 1); holotype IGP UW/A/32/305 opment in a particular lithology, and its narrow stratigraphi- (Kutek, Zeiss, 1997, pl. 15: 5) cal range. In the Middle Volga area, between Gorodischi and Characteristic ammonites. — Aulacostephanus autissio- the eastern part of the Nizhny Novgorod region (Murzicy dorensis (Cotteau) [M] (Pl. 6: 2); Aulacostephanus sp. [m]; section), the volgae horizon consists of a thin, highly biotur- A. kirghisensis (d’Orbigny) [m]; A. volgensis (Vischniakoff) bated bed of dark gray clay that is easily distinguished from [m] (Pl. 5: 9); A. subundorae (Pavlov) [m] (Pl. 5: 4, 10); the other beds in the section except for the uppermost beds of Neochetoceras cf./aff. subnudatum (Fontannes) [M] (Pl. 2: the Kimmeridgian which are lithologically similar but have 6); Lingulaticeras modestum (Ziegler) [m]; Suboxydiscites other fossil contents. The volgae horizon marks the young- sp. [M]; Sarmatisphinctes sp. [M]; ?Tolvericeras sp. [M] est mass occurrence of Nannocardioceras. Above this ho- (uncommon). rizon, a single crushed specimen of Nannocardioceras was Comments. — S. subborealis was chosen as subzonal recorded in the Gorodischi section (in the subborealis ho- index by Zeiss (2003): it is applicable to most of the Rus- rizon). Sarmatisphinctes is very rare and poorly preserved sian sections. The subborealis horizon has been referred to in the volgae horizon, mostly as inner whorls. Rogov (in: in publications (Rogov, 2005; Pimenov et al., 2005), but Pimenov et al., 2005) provisionally identified the earliest has not been described. In Russia and Poland (Kutek, Zeiss, Sarmatisphinctes of the Autissiodorensis Zone as S. magistri 1997), S. subborealis ranges through the bulk of the former and suggested that a possible magistri faunal horizon was Autissiodorensis Subzone without marked change in mor- present in the Gorodischi section. More detailed collecting phology. In the higher part of the horizon in the Volga area at this level has not yielded any well-preserved specimens and in Poland there is a thin bed rich in Neochetoceras cf./ of Sarmatisphinctes below the appearance of true S. sub- aff. subnudatum (Fontannes) (Subnudatum level of Kutek, borealis. Perisphinctids from the Eudoxus Zone that were Zeiss, 1997). This marks a short-lived event of northeast- tentatively determined as Discosphinctoides ex gr. roubya- ward mass immigration of Subtethyan ammonites to East nus (Fontannes) (Rogov, Kiselev, 2007) may belong to the Europe that can be regarded as a “migrational” faunal ho- ancestral lineage of Sarmatisphinctes. rizon (Rogov, 2005). This level is well-represented in the Correlation. — Nannocardioceras volgae (Pavlov) has upper portion of the subborealis horizon in the Murzicy sec- a narrow range in the basal Autiossiodorensis Zone through- tion. A second level rich in Submediterranean ammonites out the Subboreal Province. In Poland, Malinowska (2001) (oppeliids) occurs in both these sections in the lower part identified a wider stratigraphical range for this species rang- of the subborealis horizon. The subborealis horizon could ing into the Eudoxus Zone. However, re-evaluation of her be subdivided into at least three separate informal units on collection has shown that the stratigraphical position of at the basis of the relative frequency of occurrence of oppeliid least some of the specimens was misinterpreted. For exam- ammonites: lower and upper beds enriched in oppeliids sepa- ple, a borehole core with N. volgae and N. ewae on one side rated by an interval characterized by Subboreal perisphinc- of the sample (Malinowska, 2001, pl. 3: 1) shows a crushed tids and aulacostephanids. Sarmatisphinctes cf./aff. subborealis (Kutek and Zeiss) The perisphinctid infrazonal scale, beginning in the Up- on the opposite side and should be dated as belonging to per Kimmeridgian in the subborealis horizon, can be con- the Subborealis Subzone. The presence of rare Sutneria sub- sidered as primary for the Subboreal East-European Kim- eumela Schneid indicates that this horizon should be corre- meridgian-Volgian as based on good phylogenetic evidence. lated with part of the Subeumela Subzone of the Submediter- Kimmeridgian records of macroconchiate perisphinctids are ranean Kimmeridgian. uncommon and poorly preserved, and the scale is therefore Geographical range. — Middle Volga area (Gorodischi, based on the microconchiate succession. Murzicy sections), England (from Dorset to Wash area), Correlation. — This horizon can be correlated with northern Poland (but precise information concerning range the Boreal and Submediterranean successions using aula- of N. volgae in Poland is still absent). costephanid and Neochetoceras occurrences, respectively. However, its precise position in the Submediterranean succession is determined mainly by correlation with the subborealis horizon Zeiss, 2003 adjacent Subboreal horizons, not with the subborealis ho- (as subzone), emend. Rogov, herein rizon itself. An increase in the number of records of Neo- chetoceras near to the top of the horizon reflects climatic or/ Stratotype. — Gorodischi section, beds 9/13–9/24 (with the and biogeographic causes. The range of N. subnudatum is exception of the uppermost c. 0.13 m of bed 9/24) alternating poorly known in France; in Swabia this species has been re- light beige and dark gray clays with phosphorite nodules. corded in the hoelderi horizon of the Nusplingen Plattenkalk 110 Mikhail Rogov

(Schweigert, 1998). Probable correlations with the subbo- section (Rogov, 2005) includes, in addition to the above, realis horizon are shown in Figure 3 and Table 1. Taramelliceras cf. wepferi (Berckhemer) [M] and Glo- Geographical range. — Middle Volga area (Gorodischi, chiceras lens Berckhemer [m]. Murzicy sections), Peri-Caspian area (Elton), Orenburg area Comments. — The horizon was proposed by Rogov (Berd’anka); central Poland. (2004c) and has been referred to by Rogov (2005) and by Pimenov et al. (2005), but has not been formally described. It is present in the type area of the Fallax Subzone, but the suc- zeissi horizon Rogov, nov. (= Discosphinctoides sp. nov. cession at Gorodischi is more complete and contains a richer horizon in Pimenov et al., 2005; Rogov, 2005) ammonite assemblage, and is here chosen as the stratotype. Correlation. — The records of the Submediterranean Stratotype. — Gorodischi section, from the uppermost ammonite assemblage in the Elton section, combined with c. 0.13 m of bed 9/24 to bed 9/34, alternating light beige and a preliminary correlation using oppeliids (Rogov, 2002a), dark gray clays with uncommon phosphorite nodules and suggest that the horizon in question corresponds to the up- pyritized ammonoids. per part of the Ulmense Subzone from the zio-wepferi β to, Index species. — Sarmatisphintes zeissi Rogov, sp. probably, the hoelderi horizon (Tab. 1). This suggestion is nov. [m] (Pl. 2: 4, 5); holotype VH-17/1 (Pl. 2: 4), Gorodi- supported by the record of Sarmatisphinctes cf. fallax (Ilo- schi, 5 cm above the bottom of bed 9/25. vaisky) in the zio-wepferi β horizon of Swabia (Schweigert, Characteristic ammonites. — Aulacostephanus aff. au- 2000, pl. 2: 2), although the determination has been ques- tissiodorensis (Cotteau) [M]; A. mammatus Ziegler [m]; tioned by Scherzinger (2007): the inner whorls of S. fallax A. volgensis (Pavlov) [m]; A. cf. kirghisensis (d’Orbigny) are so variable (e.g. Pl. 5: 2, 3) that the Swabian specimen (Pl. 5: 6). might in fact belong to S. fallax. A unique record of Suboxy- Comments. — This is the only faunal horizon of the Up- discites allows correlation with some part of the Taimyrensis per Kimmeridgian in Russia that does not contain Submedi- Zone of Northern Siberia, but the full range of the species terranean and Boreal ammonoids. S. taimyrensis in the Subboreal succession is still unknown. Correlation. — Tentative, based on its position within Geographical range. — Middle Volga area (Gorodischi, the whole succession (Fig. 3, Tab. 1). Murzicy sections), Orenburg area (Berd’anka), Peri-Caspian Geographical range. — Middle Volga area (Gorodischi, area (Elton); Poland. Murzicy sections), Orenburg area (Berd’anka); ?Peri-Cas- pian area (Elton). ilowaiskii horizon Rogov, nov. Fallax Subzone Ilovaisky, 1941 (= unnamed horizon in: Rogov, 2005; in: Ilovaisky and Florensky (1941) Sarmatisphinctes sp. nov. horizon in: Pimenov et al., 2005)

fallax horizon Ilovaisky emend. Rogov, herein Stratotype. — Gorodischi section, upper part of bed 9/40 to bed 1/16, alternating light beige and dark gray clays, Stratotype. — Gorodischi section, beds 9/35 to the lower in part highly bioturbated, with phosphorite nodules. part of bed 9/40, alternating light beige and dark gray clays Index species. — Sarmatisphinctes ilowaiskii Rogov, with phosphorite nodules. sp. nov. [m] (Pl. 3: 1, 2); holotype VH-17/13 (Pl. 3: 1), Goro- Index species. — Sarmatisphinctes fallax (Ilovaisky) dischi, 10 cm above the base of bed 1/19. [m]; holotype not designated, the type succession includes Characteristic ammonites. — Aulacostephanus sp. nov. micro- and macroconchs. Two microconchs figured by Ilo- [M] (Pl. 6: 1); A. mammatus Ziegler [m]; A. jasonoides (Pav- vaisky and Florensky (1941) were recently found in the col- lov) [m]; Neochetoceras ex gr. subnudatum (Fontannes) lection of the Paleontological Institute, Moscow (person. [M]; N. rebouletianum (Fontannes) [M]; Lingulaticeras so- commun. by V.V. Mitta, June 2009). lenoides (Quenstedt) [m]. The uppermost part of this horizon Characteristic ammonites. — Aulacostephanus sp. nov. in the type section contains oppeliid ammonites (Ochetocer- [M]; A. mammatus Ziegler [m] (Pl. 2: 2, 3); A. subundorae as cf. zio (Oppel) [M], Taramelliceras cf. franciscanum (Pavlov) [m] (rare); Neochetoceras ex gr. subnudatum (Fon- (Fontannes) [M]) (Rogov, 2004c). tannes), Lingulaticeras sp., Suboxydiscites cf. taimyrensis Comments. — The possibility of the dividing the Fallax (Mesezhnikov) (extremely rare; Pl. 2: 7); an ammonite as- Subzone into two horizons was first recognised by Rogov semblage from the fallax horizon collected from the Elton (2004c). A precise ammonite biostratigraphy through the Kimmeridgian-Volgian boundary beds... 111

Correlation. — Records of Submediterranean ammonites Sokolovi Zone Ilovaisky, 1941 indicate correlation with the rebouletianum horizon (Tab. 1). in: Ilovaisky and Florensky (1941) Geographical range. — Middle Volga area (Gorodischi, Murzicy, Isady sections), Orenburg area (Berd’anka), Peri- sokolovi horizon Ilovaisky emend. Rogov, herein -Caspian area (Elton). Stratotype. — Gorodischi section, bed 1/13a, light grey Lower Volgian clay with numerous phosphorite nodules. Index species. — Ilowaiskya sokolovi (Ilovaisky) [m] (Pl. 3: 3); holotype (SD Mikhailov, 1964): Ilowaiskya The detailed ammonite biostratigraphy of the Lower Vol- sokolovi var. typica (Ilowaisky, Florenski, 1941, p. 76, pl. 8: gian has been updated by Rogov (2002b, 2004a) and Rogov et 18); Sokolovi Zone of the Sukhaya Peschanka section, Oren- al. (2006). A few new faunal horizons, including the sokolovi burg area. and pavida horizons that were proposed by Kutek and Zeiss Characteristic ammonites. — Paralingulaticeras (Rogoviceras) (1997) and Rogov (2004c), are described in this paper. cf. efimovi (Rogov) [m] (Pl. 3: 4, 5); Subdichotomoceras sp. Comments. — Ilowaiskya sokolovi and I. pavida have been shown by Kutek and Zeiss (1997) to have different Klimovi Zone Mikhailov, 1962 ranges; a possible presence of this horizon in Russia was noted by Rogov (2004 c). The lowermost part of the Klimovi Zone should be as- Correlation. — Probably with the upper part of the Hy- signed to the separate faunal horizon, but its most character- bonotum Zone (Rogov, 2004a, c) and by the occurrence of istic ammonite, Neochetoceras, is usually poorly preserved. Neochetoceras mucronatum Berckhemer et Hölder in Poland Some of the small specimens resemble N. praecursor, (Kutek, Zeiss, 1997), with ?part of the Mucronatum Zone but the precise determination of this species is a matter of (Tab. 1). The presence of Subdichotomoceras (Sphincto- controversy. ceras) suggests at least the partial correlation of this hori- Characteristic ammonites. — Neochetoceras cf. zon with the Subcrassum Zone of the Subpolar Urals and nodulosum (Berckhemer, Hölder)/N. cf. praecursor Zeiss the Scitulus-Wheatleyensis Zones of the British succession. (rare); Ilowaiskya/Sarmatisphinctes transiens [M, m]; Geographical range. — Middle Volga area (Gorodischi, Taramelliceras cf. franciscanum (Fontannes) [M]. Polevye-Bikshiki sections), Orenburg area; central Poland. Comments. — The possible presence of the horizon in ques- tion (cf. praecursor) in the Russian sections was noted by Rogov (2004c). Its correlation is tentative because of the poor preserva- pavida horizon Rogov, nov. tion of the ammonites. Oppeliids from this level resemble the uppermost Kimmeridgian (N. nodulosum, T. franciscanum) Stratotype. — Gorodischi section, bulk of the bed 1/12, and lowermost Tithonian (N. praecursor) taxa. dark grey clays, sometimes with grey interbeds. Correlation. — The Lower Tithonian eigeltingense hori- Index species. — Ilowaiskya pavida (Ilovaisky) [m]; lec- zon (Tab. 1). totype (SD Mikhailov, 1964): Ilowaiskya sokolovi var. pavi­ Geographical range. — Middle Volga area (Gorodi- da (Ilowaisky, Florenski, 1941, p. 76, pl. 12: 25); Sokolovi schi, ?Murzicy, Polevye-Bikshiki sections), Orenburg area Zone of the Sukhaya Peschanka section, Orenburg area. (Berd’anka); ?central Poland. Characteristic ammonites. — Subdichotomoceras sp.; For two overlying units, which were recognized initially “Franconites” sp. (?) (upper part of horizon in the type section). as “beds with ammonites” and afterwards as faunal horizons, Correlation. — Based on the correlation of adjacent ho- new names are proposed. There are: the steraspis horizon be- rizons (Tab. 1). low and the efimovi horizon above, which are characterized Geographical range. — Middle Volga area (Gorodischi, by acme-levels of their index species (for Paralingulaticeras Polevye-Bikshiki sections), Orenburg area (Berd’anka); cen- (Rogoviceras)4 efimovi see Pl. 5: 8). tral Poland.

4 Rogoviceras considered here as a subgenus of Paralingulaticeras because the species P. (R.) efimovi in author’s opinion directly derived from true Submediterranean Paralingulaticeras, but inhabited Subboreal Middle-Russian Sea and represented here by dwarf neotenic species showing features typical for non-mature Paralingulaticeras s.s. Perhaps such morphological shift appeared in response to unusual environments (compare with occur- rence of dwarf Amoeboceras in the Amoeboceras layers of Poland, described by Matyja, Wierzbowski, 2000). 112 Mikhail Rogov

?Pseudoscythica Zone Ilovaisky, 1941 (macroconchiate Aspidoceras cf. catalaunicum (Loriol) and in: Ilovaisky and Florensky (1941) Anaspidoceras sp. accompanied by microconchs – Sutneria) make up 37% of the assemblage (Murzicy section). “Franconites” horizon Rogov, nov. There are a few records of aspidoceratids at the same (proposed by Rogov, 2005; Pimenov et al., 2005) stratigraphical level in Poland and in England. Geyssant (1994) described two influxes of “Sutneria rebholzi” at Stratotype. — Gorodischi section, uppermost part of bed the beginning of the Autissiodorensis Zone in Yorkshire. 1/12 and lowermost part of bed 1/11, highly bioturbated dark In addition, rare records of A. cf. catalaunicum (Loriol) from grey clays and light beige clays. the Pechora area (Mesezhnikov, 1984, pl. 32: 1) and the Sub- Index species. — Horizon determined by the presence polar Urals (referred to as A. longispinum by Zakharov et al., of ammonites resembling the Submediterranean Franconites. 2005) show how extensive spread of these ammonites was, Characteristic ammonites. — ?Franconites cf. vimineus possibly as the result of a short-lived warmer climate. Car- (Schneid) (Mikhailov, 1964, pl. 9: 1; Rogov, 2005, fig. 4, dioceratids are represented at this stratigraphical level by the refigured at Pl. 4: 1 herein) [M]; Franconites? sp. wide-ranging species N. krausei (Salfeld) and possibly by N. Comments. — The ?Franconites figured from the Rus- cf. ewae (Malinowska), although records of the latter species sian Platform by Mikhailov (1964) and Mitta (2004) were re- are not numerous. -determined as Ilowaiskya by Scherzinger and Mitta (2006). The volgae horizon represents the last mass occurrence Additional sampling of this stratigraphical level is necessary of cardioceratids throughout the Middle Volga area and west- before further progress can be made. wards from there as far as England. It is the youngest level in Geographical range. — Middle Volga area (Gorodischi, the Kimmeridgian of European Russia in which Boreal am- Polevye-Bikshiki sections), Orenburg area (Berd’anka). monoids predominate. The next mass immigration of Boreal ammonites occurred at the beginning of the Middle Volgian. Interestingly, the lithology, thickness and ammonite assem- Brief review of oscillations in the ammonite blages of the volgae horizon are closely similar throughout assemblages through the Kimmeridgian- the Middle Volga area. -Volgian transition of the Middle Volga area Most of the Upper Kimmeridgian ammonite assemblages above the volgae horizon are characterized by a predomi- nance of the Subboreal Sarmatisphinctes and Aulacostepha- The ammonite assemblages in the Kimmerdigian-Vol- nus. The abundance of the Submediterranean taramelli- gian transition beds consist of mixtures of Subboreal, Boreal ceratid Neochetoceras shows remarkable oscillations. The and Submediterranean taxa. The Lower Volgian assemblages lower and upper parts of the subborealis horizon are charac- were shown by the present author (Rogov, 2004c) to contain terized in Russia by a high Neochetoceras contents: the up- oscillating compositions, but the Upper Kimmeridgian part per level has a well-documented distribution as far as central of the Gorodischi section has only been intensively exca- Poland (the Subnudatum level of Kutek, Zeiss, 1997) which vated since 2005. The Upper Kimmeridgian ammonite as- may reflect a climatic change. semblages have now been shown to be characterized also The Fallax Subzone is characterized by a steady upward by changes in the relative abundances of ammonites of Sub- increase in Submediterranean influence that has been well mediterranean, Subboreal and Boreal affinities (Fig. 3C). documented throughout the Middle Volga area and up into The highest part of the Eudoxus Zone (cf. anglicum ho- the Lower Volgian (see Rogov, 2004c). Boreal ammonites, rizon) is characterized by the mass occurrence of the Bo- including very rare Suboxydiscites cf. taimyrensis (Mesezh- real ammonite Nannocardioceras. A similar event has been nikov), suggest that the Taimyrensis Zone of Northern Si- recorded in England (Nannocardioceras Beds) and Poland beria is the correlative of the whole Autissiodorensis Zone. where it correspond in part to the Amoeboceras Beds of Ma- The beginning of the Lower Volgian is marked by linowska (2001). Beds with Nannocardioceras in the upper the maximum proportion of Submediterranean ammonites part of the Eudoxus Zone of the Subpolar Urals (Zakharov et in the assemblages of the Middle Volga area. This propor- al., 2005) and Northern Siberia (Mesezhnikov, 1984) repre- tion then gradually decreases as the Boreal Eosphinctoceras sent correlatives of this faunal event. and Subdichotomoceras become increasingly abundant. The The beginning of the Autissiodorensis Zone (aff. rebholzi pavida horizon contains a poor assemblage that includes horizon) is marked by a conspicuous change in the ammo- a mixture of Boreal (Subdichotomoceras) and Subboreal nite assemblage. Aulacostephanids (including two morpho- (Ilowaiskya) ammonites, Boreal belemnites, and Boreal bi- types of A. autissiodorensis with smooth and ribbed termi- valves including Buchia and oysters (Rogov, 2005). nal body chambers) become numerous, and aspidoceratids A precise ammonite biostratigraphy through the Kimmeridgian-Volgian boundary beds... 113

The “Franconites” horizon contains a few Franconites- Description. — S. zeissi has dense biplicate ribbing with -like ammonites accompanied by badly preserved Ilowaiskya a few triplicate ribs in the mature stages of ontogenic de- of the pseudoscythica group. Another mass immigration of velopment not connected directly to constrictions. Innermost Submediterranean ammonites occurred in the neoburgense whorls usually covered by bifurcate ribs. Prorsiradiate con- hemera (Rogov, 2004a, 2005) when Anaspidoceras neobur- strictions are not numerous and are usually weakly devel- gense became the most numerous ammonite in the Gorodi- oped. They are mostly followed by a simple rib, and pre- schi area. This horizon has been recognized in other regions ceded by a compound bidichotome (or sometimes triplicate) of European Russia, including the Belgorod and Moscow rib. The ribs are rusiradiate at the umbilical edge, becoming areas, where that ammonite occurs re-deposited in phospho- rectiradiate to prorsiradiate near the whorl-flanks. The ribs rite moulds, and in the Orenburg area (Berd’yanka section). furcate relatively higher than in S. subborealis, in the up- In the Middle Volga area these ammonites are accompanied per half to upper third of the whorl side. Ribs may furcate by numerous belemnites of Submediterranean origin (Hibo- in a symmetrical or asymmetrical fashion. In the latter case lithes, see Ippolitov, 2006; Rogov et al., 2006), which are the anterior secondary rib approximately follows the course smaller in size than their counterparts in Tethys. of the primary rib and the anterior rib diverges backwards Another marked change in the ammonite assemblages oc- from this direction. An unstable, irregular mode of furcation curs at the beginning of the puschi horizon where the ammo- (a typical feature of Sarmatisphinctes) is represented in S. nites are mainly Subboreal (“Pseudovirgatites” derived from zeissi by the difference in the neighbouring rib furcation. In Ilowaiskya: possibly homoeomorphs of the true Submediterra- addition, the rib density, furcation point and character of the nean Pseudovirgatites). Possible Submediterranean perisphinc- constrictions are highly variable in this species in compari- tids are uncommon at this stratigraphical level where they are son with other forms of Sarmatisphinctes. The sizes of adults represented by Danubisphinctes sp. (Rogov, 2004c). in this genus shows significant variation, up to at least two diameters. Discussion. — Sarmatisphinctes zeissi is distinguished Descriptions of new ammonite species from the closely similar form S. subborealis by the presence of triplicate ribs which are not connected to constrictions. In Sarmatisphinctes zeissi Rogov, sp. nov. [m] addition, the rib density (estimated from the number of pri- mary ribs per quarter whorl) is usually significantly higher (Pl. 2: 4, 5) in S. zeissi than in S. subborealis. Interestingly, in the Goro- 1963 Subplanites pseudoscythicus (Ilovaisky et Florensky); dischi and Murzicy sections (c. 200 km apart) the first rep- Pachucki p. 9, pl. 3: 4. resentatives of S. zeissi appear at the same level which is 1997 Discosphinctoides subborealis sp. nov; Kutek and Zeiss, developed in the same lithology in the lithostratigraphical p. 139, pl. 18: 3. succession. The first appearance of S. subborealis is in the Holotype: Specimen VH-17/1, Vernadsky State Geological Mu- uppermost part of a light grey bed at the base of alternating seum of RAS (Pl. 2: 4). dense dark grey and grey clays. S. zeissi also resembles S. Type locality: Gorodischi section, Ulianovsk Volga area, 5 cm fallax of the ancient morphotype, but in the latter species above the bottom of bed 9/25. tripartite ribs (sometimes with virgatotome ribs) are much Type horizon: Upper Kimmeridgian, Autissiodorensis Zone, more numerous and the rib density is lower in the outer Subborealis Subzone, zeissi horizon. whorls. S. fallax, S. zeissi and S. subborealis appear to be Derivation of name: In honor of Dr. Arnold Zeiss (Erlangen, closely related forms that belong to the same lineage. Germany), whose research greatly improved our understanding Geographical and stratigraphical range. — Zeissi hori­zon of the ammonite assemblages and biostratigraphy of the Kim- of the Middle Volga area (Gorodischi, Murzicy sections), meridgian-Volgian boundary beds. Diagnosis: Microconchs showing features intermediate between Orenburg area (Berd’anka); Peri-Caspian area (Elton) and S. subborealis and S. fallax. S. zeissi has some triplicate ribs, Polish Lowland. but these are not connected to constrictions; constrictions are usually bounded by simple and dichotomitic ribs. Aperture with small lappets. Sarmatisphinctes ilowaiskii Rogov, sp. nov. [m]

Material. — Gorodischi section: VH-17/1, VH-17/2, (Pl. 3: 1, 2) VH-17/3, VH-17/4, VH-17/5, VH-17/6, VH-17/7, VH-17/8, 1963 Zaraiskites pilicensis (Michalsky); Pachucki, p. 8, pl. 3: 5, 6. VH-17/51, VH-17/55; Murzicy section: VH-17/9, VH-17/10; 1963 Zaraiskites scythicus (Michalsky); Pachucki, p. 8, pl. 4: Berd’yanka section: VH-17/12. 2 (only). 114 Mikhail Rogov

1973 Subplanites klimovi (Ilovayski et Florenski); Dembowska, Diagnosis: Microconchs having closely similar ornamentation p. 99, pl. 1: 5, 6. to that of the earliest Ilowaiskya from which they are distin- 1973 Subplanites kokeni (Behrendsen); Dembowska, p. 99, pl. 2: 1. guished by more unstable ribbing. Sarmatisphinctes usually has 1973 Subplanites cf. kokeni (Behrendsen); Dembowska, p. 99, biplicate and triplicate ribs in more or less equal proportion, with pl. 2: 2, 5. virgatotome ribs in some specimens. Triplicate ribs mostly ap- 1973 Subplanites sp.; Dembowska, p. 100, pl. 2: 3. pear at the diameters of 3 to 5 cm. Aperture with small lappets. 1997 Sarmatisphinctes fallax (Ilovaisky); Kutek and Zeiss, p. 145, pl. 20: 1; pl. 21: 1, 3, 4; pl. 22: 1–6. Material. — Gorodischi section: VH-17/13, VH-17/15, 2002a Sarmatisphinctes fallax (Ilovaisky); Rogov, pl. 1: 6. 2004b Sarmatisphinctes sp. nov.; Rogov, pl. 1: 2, 3. VH-17/16, VH-17/17; Berd’yanka section: VH-17/19, VH- 17/20, VH-17/21; Elton section: VH-17/22, VH-17/23; Holotype: Specimen VH-17/13 (Pl. 3: 1), Vernadsky State Geo- Isady section: VH-17/24, VH-17/25, VH-17/26, VH-17/27, logical Museum of RAS. VH-17/28; Murzicy section: VH-17/14, VH-17/29, VH- Type locality: Gorodischi section, Ulianovsk Volga area, bed 17/30, VH-17/31. 1/19, 10 cm above the base. Type horizon: Upper Kimmeridgian, Autissiodorensis Zone, Description. — The ribbing pattern consists of dense Fallax Subzone, ilowaiskii horizon. biplicate and triplicate ribs in different ratios. In the outer Derivation of name: In honor of David Ilowaisky, distinguished whorls it varies from ~equal to a prevalence of either bipli- Russian paleontologist who carried out detailed studies of the cate or triplicate ribs with additional virgataxioceratid ribs Kimmeridgian-Volgian boundary beds of the Orenburg area. and simple ribs. The ribbing in the innermost whorls is pre- dominantly bifurcate, but tripartite ribs appear early in the shell ontogeny in some specimens. Prorsiradiate constric- tions are not common and are sometimes absent. They are usually followed by a simple rib, and preceded by a com- A n = 50 pound bidichotome (or sometimes triplicate) rib. The ribs are rusiradiate at the umbilical edge, becoming rectiradiate to 2.9 prorsiradiate on the whorl-flank. The ribs furcate approxi- 2.7 mately at the middle of the flanks, but the furcation point is 2.5 variable and can be located in the lower or upper half of the 2.3 rr/4 whorl. Ribs may furcate in a symmetrical or asymmetrical 2.1 fashion; in the latter case the anterior secondary rib follows 1.9 approximately the course of the primary rib and the ante-

1.7 rior rib diverges backwards. An irregular mode of furcation

1.5 frequently appears in the rib succession: biplicate, triplicate, subborealis zeissi fallax ilowaiskii simple and virgatotome ribs alternate in the different modes. The evolution of the Sarmatisphinctes lineage shows major B n = 50 11.0 alternations in the principal ornamentation features such 10.5 as secondary/primary rib ratio and density of primary ribs 10.0 (Figs 4, 5). The same phenomenon is present in the evolu- 9.5 tionary successions of the Keppleritinae (Callomon, 2004; 9.0 Kiselev, Rogov, 2007) and the Cadoceratinae (Kiselev,

pr/4 8.5 Rogov, 2007). In consequence it is easy to confuse ammo- 8.0 nites of different ages with one another. When collected from 7.5 loose blocks, the specimens of Sarmatisphinctes zeissi and 7.0 S. ilowaiskii could be misidentified as belonging to a single 6.5 species. Both, however, can be separated from S. fallax with- 6.0 subborealis zeissi fallax ilowaiskii out difficulty. Discussion. — It is difficult to distinguish S. ilowaiskii Fig. 4. Change of primary rib density and secondary/ from progressive and ancestral morphotypes of S. fallax. primary rib ratio in the evolution of the Sarmatisphinctes The principal distinction of the species is a rarity of virga- subborealis – S. zeissi – S. fallax – S. ilowaiskii lineage totome ribs and the prevalence of biplicate and triplicate ribs 1 A – variations in the average rib ratio per /4 whorl (rr/4); B – variations in in S. ilowaiskii. This species is also closely related to the ear- 1 the average rib density per /4 whorl (pr/4); n – number of specimens liest Ilowaiskya (I. klimovi) from which it differs in having A precise ammonite biostratigraphy through the Kimmeridgian-Volgian boundary beds... 115

4.0

3.5

3.0 S. ilowaiskii

S. fallax

rib ratio/4 2.5 S. zeissi 2.0 S. subborealis

1.5 5 6 7 8 9 10 11 12 13 rib density/4

Fig. 5. Evolutionary changes in the Sarmatisphinctes subborealis – S. zeissi – S. fallax – S. iloiwaiskii lineage Transparent arrows show the trend of the changes (cf. Fig. 4) in the principal features more irregular ribbing and an earlier appearance of triplicate English uppermost Kimmeridgian were made by R. Gallois ribs. The latter usually appear at half of the diameter at which (Exeter), who also checked spelling of the text. they occur in Ilowaiskya. Studied ammonites are stored in the Vernadsky Geologi- Geographical and stratigraphical range. — Ilowaiskii ho- cal Museum of RAS, Moscow (VH), and CNIGR Museum, rizon of the Middle Volga area (Gorodischi, Murzicy, Isady Saint-Petersburg (CNIGR). sections); Orenburg area (Berd’anka); Peri-Caspian area (El- ton); central Poland. References Acknowledgements. This study has been supported by RFBR grants 06-05-64284, 05-05-65262, 09-05-00456 Berckhemer F., Hölder H., 1959 — Ammonites aus dem Russian Science Support Foundation and by grant of the Oberen Weißen Jura Süddeutchland. Beihefte zum Geolo- President of the Russian Federation МК-856.2008.5. I also gisches Jahrbuch, 35: 1–135. warmly thank V.V. Mitta (Moscow), F. Atrops (Lyon) and Callomon J.H., 2004 — Appendix. Description of a new species A. Scherzinger (Hattingen) for their valuable comments of ammonite, Kepplerites tenuifasciculatus n. sp. from the Middle and discussions, and to my colleagues who helped me in Jurassic, Lower Callovian of East Greenland. Bulletin of the Geo- the fieldwork in the Middle Volga area during the last few logical Survey of Denmark and Greenland, 5: 42–49. years: A. Guzhov, A. Manikin, M. Pimenov (Saratov), A. Callomon J.H., Cope J.C.W., 1971 — The stratigraphy and Guzhov, L. Ermakova, A. Ippolitov, A. Orlova, V. Piskunov, ammonite succession of the Oxford and Kimmeridge clays in E. Schepetova, E. Terekhova (Moscow), A. Sapozhenkov the Warlingham Borehole. Bulletin of the Geological Survey (Kostroma), O. Dzyuba (Novosibirsk) and D. Kiselev (Yaro- of Great Britain, 36: 147–177. slavl). Additional valuable suggestions were made by two Callomon J.H., Cope J.C.W., 1995 — The Jurassic geology reviewers, G. Schweigert (Stuttgart) and J. Kutek (Warsaw), of Dorset. In: Field geology of the British Jurassic (ed. P.D. who also introduced the author to the collections of Kim- Taylor): 51–103. Geological Society, London. meridgian, Tithonian and Volgian ammonites from Swabia Cope J.C.W., 1996 — The role of the Secondary Standard in and Poland. The collections of L. Malinowska, stored in the stratigraphy. Geological Magazine, 133: 107–110. Polish Geological Institute, were made available through the Cox B.M., Gallois R.W., 1981 — The stratigraphy of the Kim- friendly help of T. Woroncowa-Marcinowska, W. Mizerski meridge Clay of the Dorset type area and its correlation with and H. Wierzbowski (Warsaw); while access to collections some other Kimmeridgian sequences. Report of the Institute of Kimmeridgian ammonites from southern England stored of Geological Sciences, 80, 4: 1–44. in the British Museum (Natural History) was organized Dembowska J., 1973 — Portlandian in the Polish Lowlands. through the help of W. Wimbledon (Bristol), A. McGowan Prace Instytutu Geologicznego, 70: 1–107 [in Polish with and S. Nikolaeva (London). Valuable remarks concerning ­English sum.]. 116 Mikhail Rogov

Gallois R.W., 2000 — The stratigraphy of the Kimmeridge Clay the Aulacostephanus eudoxus Zone (Upper Kimmeridgian) in Formation (Upper Jurassic) in the RGGE Project boreholes at northern and central Poland. Biuletyn Państwowego Instytutu Swanworth Quarry and Metherhills, south Dorset. Proceedings Geologicznego, 397: 5–64 [in Polish with English sum.]. of the Geologists’ Association, 111: 265–280. Matyja B.A., Wierzbowski A., 2000 — Biological response Gerasimov P.A., Mikhailov N.P., 1966 — Volgian Stage of ammonites to changing environmental conditions: an exam- and International Stratigraphical Scale of the Upper Jurassic ple of Boreal Amoeboceras invasions into Submediterranean Series. Proceedings of the Russian Academy of Sciences, Geo­ Province during Late . Acta Geologica Polonica, 50, logy, 2: 118–138 [in Russian]. 1: 45–54. Geyer O.F., 1969 — The ammonite genus Sutneria in the Upper Mesezhnikov M.S., 1984 — Kimmeridgian and Volgian Stag- Jurassic of Europe. Lethaiа, 2, 1: 69–72. es of north of the USSR: 1–224, Nedra, Leningrad [in Russian]. Geyssant J., 1994 — Colonisation par des ammonites méri- Mikhailov N.P., 1962 — Upper boundary of the Kimmeridgian dionales des mers subboréales kimméridgiennes du Yorkshire Stage. Proceedings of the Academy of Sciences of USSR, 145, (Angleterre). Geobios, Mémoire Special, 17: 245–254. 6: 1366–1368 [in Russian]. Hantzpergue P., 1989 — Les ammonites kimméridgiennes Mikhailov N.P., 1964 — Boreal Late Jurassic (Lower Volgian) du haut-fond d’Europe occidentale. Biochronologie, Systéma- ammonites (Virgatosphinctinae). Transactions of the Geologi- tique, Evolution, Paléobiogéographie. Cahiers de Paléontolo- cal Institute of the Academy of Sciences of USSR, 107: 7–88 gie, 428 pp. [in Russian]. Ilovaisky D.I., Florensky K.R., 1941 — Les ammonites du Mitta V.V., 2004 — On the new publications on Jurassic ammo- Jurassique supérieur des bassins des rivières Oural et Ilek. Con- nites and stratigraphy. Bulletin of the Society of Naturalists of tribution a la Connaissance de la Géologie de l’USSR. Nou- Moscow, Section Geology, 79, 1: 90–98 [in Russian]. velle Série, 1, 5: 7–195 [in Russian]. Pachucki C., 1963 — Die Ammoniten-Fauna des unteren Ippolitov A.P., 2006 — On the possible expression of sexual Bononiens und des oberen Kimmeridge in Bełchatów und dimorphism in Hibolithes Montfort, 1808 from the Middle and Tuczyn. Annales Universitatis Mariae Curie-Skłodowska, Upper Jurassic of European Russia. In: Contributions to cur- Lublin – Polonia, Sectio B, 18, 1: 1–21 [in Polish with German rent research: morphology, systematics, evolution, sum.] ecology and biostratigraphy (eds. I.S. Barskov, T.B. Leonova): Pavlov A.P., 1886 — Les ammonites de la zone à Aspido­ 57–60. Paleontological Institute of RAS, Moscow [in Russian]. ceras acanthicum de l’est de la Russie. Mémoires du Comité Khudyaev I., 1932 — The fauna of the Upper Kimmeridgian Géologique, 2, 3: 1–91. deposits of Timan. Bulletin of the United Geological and Pros- Pimenov M.V., Guzhikov A.Yu., Rogov M.A., 2005 — Pre- pecting Service, 51, 42: 645–654 [in Russian]. liminary results of the magnetostratigraphic study of the Up- Kiselev D.N., Rogov M.A., 2005 — Infrazonal stratigraphy and per Kimmeridgian-Volgian section (Gorodischi, Ulianovsk ammonites of the Middle-Upper Volgian boundary beds of the area). In: Materials of the first All-Russian Meeting “Jurassic European Russia. In: Materials of the first All-Russian Conference System of Russia: problems of stratigraphy and paleogeogra- “Jurassic System of Russia: problems of stratigraphy and phy” (eds. V.A. Zakharov et al.): 191–192. Geological Institute paleogeography” (eds V.A. Zakharov et al.): 135–139. Geological of RAS, Moscow [in Russian]. Institute of RAS, Moscow [in Russian]. Rogov M.A., 2002a — Autissiodorensis Zone (Upper Kimmer- Kiselev D.N., Rogov M.A., 2007 — Stratigraphy of the Ba- idgian) of the Volga area: ammonite assemblages, biostratigra- thonian–Callovian boundary deposits in the Prosek Section phy, correlation. In: Modern questions of geology: 320–325. (Middle Volga Region). Article 1. Ammonites and infrazonal Nauchny Mir, Moscow [in Russian]. biostratigraphy. Stratigraphy and Geological Correlation, 15: Rogov M.A., 2002b — Stratigraphy of Lower Volgian deposits 485–515. in the Russian Plate and correlation between Volgian and Ti- Krause P.G., 1908 — Über Diluvium, Tertiär, Kreide und Jura in thonian Stages. Stratigraphy and Geological Correlation, 10, der Heilsberger Tiefbohrung. Jahrbuch der Könglischen Preuß- 4: 348–364. sischen Geologischen Landesanstalt, 29, 2: 185–325. Rogov M.A., 2004a — Ammonite-based correlation of the Lower Kutek J., 1961 — Kimeryd i bonon Stobnicy (Le Kiméridgien and Middle (Panderi Zone) Volgian Substages with the Tithonian et la Bononien de Stobnica). Acta Geologia Polonica, 11, 1: Stage. Stratigraphy and Geological Correlation, 12, 1: 35–57. 103–183 [in Polish with French sum.] Rogov M.A., 2004b — New data on stratigraphy of the Kim- Kutek J., Zeiss A., 1997 — The highest Kimmeridgian and meridgian-Volgian boundary beds of the Russian Platform. Lower Volgian in Central Poland; their ammonites and biostra- In: Problems of regional geology: museum perspective (eds. tigraphy. Acta Geologica Polonica, 47, 3/4: 107–198. G.V. Kalabin et al.): 174–176. Acropol, Мoscow [in Russian]. Malinowska L., 2001 — Ammonites of the genera Amoebo- Rogov M.A., 2004c — The Russian Platform as a key region for ceras Hyatt, 1900, and Enosphinctes Schindewolf, 1925 from Volgian/Tithonian correlation: A review of the Mediterranean A precise ammonite biostratigraphy through the Kimmeridgian-Volgian boundary beds... 117

faunal elements and ammonite biostratigraphy of the Volgian In: Advances in Jurassic Research 2000 (eds. R.L. Hall, stage. Rivista Italiana di Paleontologia e Stratigrafia, 110, 1: P.L. Smith). GeoResearch Forum, 6: 195–202. 321–328. Schweigert G., Krishna J., Pandey B., Pathak D.B., Rogov M.A., 2005 — Molluscan associations of the Late Juras- 1996 — A new approach to the correlation of the Upper Kim- sic Seas of the East-European Platform. Transactions of the meridgian Beckeri Zone across the Tethyan Sea. Neues Jahr- Geological Institute of the Russian Academy of Sciences, 516: buch für Geologie und Paläontologie, Abhandlungen, 202: 178–199 [in Russian]. 345–373. Rogov M.A., Kiselev D.N., 2007 — The Kimmeridgian Thierry J., 2000 — Early Tithonian. In: Atlas Peri-Tethys. of Russia and adjacent areas, its subdivision and correlation. Palaeogeographical maps (coord. S. Crasquin). Explanatory Unpublished field trip guidebook. Geological Institute of RAS, notes: 99–110, CCGM/CGMW, Paris. Moscow. Van der Vyver C.P., 1986 — The stratigraphy and ammonite Rogov M., Schepetova E., Ustinova M., Price G.D., faunas of the Lower Kimmeridgian of Britain [unpublished Guzhikov A., Pimenov M., Dzyuba O., 2006 — PhD thesis]. University of Wales, Cardiff. A multi-proxy study of the Kimmeridgian/Volgian boundary Zakharov V.A., 2003 — In defence of the Volgian Stage. beds in the Gorodischi section (Middle Volga area, Russia), Stratigraphy and Geological Correlation, 6: 585–593. the lectostratotype of the Volgian Stage. Volumina Jurassica, Zakharov V.A., Baudin F., Daux V., Dzyuba O.S., 4: 208–210. Zverev K.V., Renard M., 2005 — Isotopic and faunal Scherzinger A., 2007 — New studies on dimorphism and phy- record of high paleotemperatures in the Kimmeridgian of Sub- logeny in Virgataxioceras Arkell (; Late Jurassic). polar Urals. Russian Geology and Geophysics, 46, 1: 1–18. Newsletter of the International Subcommission on Jurassic Zakharov V.A., Rogov M.A., 2003 — Boreal-Tethyan mol- Stratigraphy, 34, 2: 24. lusk migrations at the Jurassic-Cretaceous boundary time and Scherzirger A., Mitta V.V., 2006 — New data on ammo- biogeographic ecotone position in the Northern Hemisphere. nites and stratigraphy of the Upper Kimmeridgian and Lower Stratigraphy and Geological Correlation, 11, 2: 152–171. Volgian (Upper Jurassic) of the middle Volga Region (Russia). Zeiss A., 1979 — Neue Sutnerien-Funde aus Ostafrika ihre Neues Jahrbuch für Geologie und Paläontologie, Abhandlun- ­Bedeutung für Taxonomie und Phylogenie der Gattung. gen, 214: 225–251. Paläonto­logische Zeitschrift, 53, 3/4: 259–280. Scherzinger A., Schweigert G., 2003 — Ein Profil in der Zeiss A., 1994 — Neue Ammonitenfunde aus dem oberen Malm Usseltal- und Rennertshofen-Formation der südlichen Frank- Süddeutschlands. Abhandlungen der Geologischen Bundesan- enalb (Unter-Tithonium). Zitteliana. Reihe A, 43: 3–16. stalt, 50: 509–528. Schweigert G., 1998 — Die Ammonitenfauna des Nusplinger Zeiss A., 2003 — The Upper Jurassic in Europe: its subdivision Plattenkalks (Ober-Kimmeridgium, Beckeri-Zone, Ulmense- and correlation. Bulletin of the Geological Survey of Denmark Subzone, Baden-Württemberg). Stuttgarter Beiträge zur and Greenland, 1: 75–114. Naturkunde. Serie B (Geologie und Paläontologie), 267: 1–61. Ziegler B., 1962— Die Ammoniten-Gattung Aulacostephanus Schweigert G., 2000 — New biostratigraphic data from the im Oberjura (Taxionomie, Stratigraphie, Biologie). Palaeonto- Kimmeridgian/Tithonian Boundary Beds of SW Germany. graphica, Abteilung A, 119: 1–172.

PLATES Plate 1

Fig. 1. Nannocardioceras cf. anglicum (Salfeld, 1915) [m] VH-17/32; Gorodischi, 2.25 m above the base of the bed 9/2; Upper Kimmeridgian, Eudoxus Zone, cf. anglicum horizon; × 2 Figs 2–3. Nannocardioceras krausei (Salfeld, 1915) [m] 2 – specimen lost. Murzicy, bed 2; 3 – VH-17/33; Gorodischi, bed 9/9; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, aff. rebholzi horizon; × 2 Figs 4–6. Nannocardioceras volgae (Pavlov, 1886) [m] 4 – lectotype, CNIGR 37/312 (= Pavlov, 1886, pl. 8: 5); 5 – VH-17/34; 6 – VH-17/11; bed 9/10; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, volgae horizon; × 2 Figs 7–9. Sutneria aff. rebholzi Berckhemer, 1922 [m] 7 – VH-17/35, 8 – VH-17/36, 9 – VH-17/37; Murzicy, bed 1/2; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, aff. rebholzi horizon; 7, 8 – × 2; 9 – natural size Figs 10. Neochetoceras cf. subsidens (Fontannes, 1879) [M] specimen lost; Gorodischi, bed 9/9; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, aff. rebholzi horizon; natural size Figs 11–12. Sarmatisphinctes subborealis (Kutek et Zeiss, 1997) [m] 11 – VH-17/38, 12 – VH-17/39; Gorodischi, 1 m below the top of the bed 9/24; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, subborealis horizon; natural size

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Mikhail ROGOV — A precise ammonite biostratigraphy through the Kimmmeridgian-Volgian boundary beds... PLATE 2

Fig. 1a–c. Aulacostephanus volongensis Khudyaev 1932 [m] holotype CNIGR 2/3434 (= Khudyaev, 1932, pl. 1: 3); glauconitic clay of Volonga river; Upper Kimmeridgian Figs 2–3. Aulacostephanus sp. nov. aff. mammatus Ziegler, 1962 [m] 2 – VH-17/40; Murzicy, 2.3 m above the base of bed 14; 3 – VH-17/41, Gorodischi, bed 9/37; Upper Kimmeridgian, Autissiodorensis Zone, Fallax Subzone, fallax horizon Figs 4–5. Sarmatisphinctes zeissi Rogov sp. nov. [m] 4 – holotype VH-17/1, 5 – VH-17/8; Gorodischi, bed 9/25; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, zeissi horizon Fig. 6. Neochetoceras cf. subnudatum (Fontannes, 1879) [M] VH-17/42; Gorodischi, 6.95 m above the base of bed 9/2; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, subborealis horizon Fig. 7. Suboxydiscites cf. taimyrensis (Mesezhnikov, 1976) [M] VH-17/43; Gorodischi, 1.65 m below the top of the bed 9/41; Upper Kimmeridgian, Autissiodorensis Zone, Fallax Subzone, fallax horizon Fig. 8. Aspidoceras cf. catalaunicum (Loriol, 1872) [M] VH-17/44; Murzicy, beds 1–2; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, aff. rebholzi horizon

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Mikhail ROGOV — A precise ammonite biostratigraphy through the Kimmmeridgian-Volgian boundary beds... PLATE 3

Figs 1–2. Sarmatisphinctes ilowaiskii Rogov sp. nov. [m] 1 – holotype VH-17/13, Gorodischi, 0.1 m above the base of bed 1/19; 2 – VH-17/17, Gorodischi, bed 1/16; Upper Kimmeridgian, Autissiodorensis Zone, Fallax Subzone, ilowaiskii horizon Fig. 3. Ilowaiskya sokolovi (Ilovaisky, 1941) [m] VH-17/45; Gorodischi, bed 1/13a; Lower Volgian, Sokolovi Zone, sokolovi horizon Figs 4–5. Paralingulaticeras (Rogoviceras) cf. efimovi (Rogov, 2002) [m] 4 – VH-17/46, 5 – VH-17/47; Gorodischi, bed 1/13a; Lower Volgian, Sokolovi Zone, sokolovi horizon; × 2

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Mikhail ROGOV — A precise ammonite biostratigraphy through the Kimmmeridgian-Volgian boundary beds... PLATE 4

Fig. 1. ?Franconites sp. Photographed in situ in the field at Gorodischi, bed 1/10; Lower Volgian, ?Pseudoscythica Zone, “Franconites” horizon; × 0.5 Volumina Jurassica, VIII PLATE 4

Mikhail ROGOV — A precise ammonite biostratigraphy through the Kimmmeridgian-Volgian boundary beds... PLATE 5

Fig. 1. Sarmatispinctes subborealis (Kutek et Zeiss, 1997) VH-17/48; Murzicy, 0.1 m below the top of the bed 13; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, subborealis horizon Figs 2–3. Sarmatisphinctes fallax (Ilovaisky) 2 – VH-17/49, Murzicy, 2 m above the base of the bed 14; 3 – VH-17/50, Murzicy, 2.1 m above the base of the bed 14; Upper Kimmeridgian, Autissiodorensis Zone, Fallax Subzone, fallax horizon Figs 4, 10. Aulacostephanus subundorae (Pavlov) [m] 4 – specimen lost, Gorodischi, 7.45 m above the base of the bed 9/2; 10 – ВХ-17/52, Gorodischi, 1 m below the top of the bed 9/24; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, subborealis horizon (“Taramelliceras level”) Fig. 5. Aulacostephanus kirghisensis (d’Orbigny) [m] VH-17/53; Murzicy, bed 2; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, aff. rebholzi horizon Fig. 6. A. cf. kirghisensis (d’Orbigny) [m] VH-17/54; base of the bed 9/25; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, zeissi horizon Fig. 7. Sutneria aff. rebholzi Berckhemer [m] specimen lost; Gorodischi, 0.1 m above the base of the bed 9/9; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, aff. rebholzi horizon; × 2 Fig. 8. Paralingulaticeras (Rogoviceras) efimovi (Rogov) [m] VH-17/18; Lower Volgian, Klimovi Zone; × 2 Fig. 9. Aulacostephanus cf. volgensis (Vischniakoff) VH-17/52; Gorodschi, 1 m below the top of the bed 9/24; Subborealis Subzone, subborealis horizon (“Taramelliceras level”)

All ammonites are coated with ammonium chloride, figured in natural size except Fig. 7, 8, which are enlarged two times Volumina Jurassica, VIII PLATE 5

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Mikhail ROGOV — A precise ammonite biostratigraphy through the Kimmmeridgian-Volgian boundary beds... PLATE 6

Fig. 1. Aulacostephanus sp. nov. [M] Gorodischi, photographed in the field (2005), 1 m below the top of the bed 1/19; Upper Kimmeridgian, Autissiodorensis Zone, Fallax Subzone, ilowaiskii horizon; × 0.4 Fig. 2. Aulacostephanus autissiodorensis (Cotteau) [M] Murzicy, photographed in the field (2006), 0.1 m above the base of the bed 5; Upper Kimmeridgian, Autissiodorensis Zone, Subborealis Subzone, subborealis horizon; × 0.4 Volumina Jurassica, VIII PLATE 6

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Mikhail ROGOV — A precise ammonite biostratigraphy through the Kimmmeridgian-Volgian boundary beds...